1. Field of the Invention
The present invention relates to an engine control system provided with a NOx purifying facility purifying NOx in exhaust gases, and more particularly to an engine control system which injects fuel in a main injection mode or a pilot-and-main injection mode.
2. Description of the Related Art
Usually, NOx in exhaust gases emitted from an internal combustion engine is purified by a NOx purifying facility. For instance, a NOx purifying facility for a diesel engine is constituted by a catalytic converter containing NOx catalyst and provided in an exhaust system and selectively purifying NOx, and an urea solution supply arranged upstream of the catalytic converter.
The NOx catalyst includes a catalytic carrier carrying vanadium oxide (V2O2) to which urea solution is supplied as a reducing agent, and purifies NOx in an atmosphere filled with excess oxygen.
The hydrolysis and pyrolysis of the urea solution is expressed by the following formula. Hydrolyzed and pyrolyzed urea solution effuses NH3.(NH2)2CO+H2O→2NH3+CO2  (1)
The denitration of NH3 and nitrogen oxide by the NOx catalyst is expressed by formulas (2) and (3).4NH3+4NO+O2→4N2+6H2O  (2)2NH3+NO+NO2→2N2+3H2O  (3)
Referring to FIG. 5 of the accompanying drawings, the purifying efficiency of the NOx purifying facility to which ammonia (or urea solution) is added is high when a catalyst temperature is approximately 350° C., but is abruptly reduced at temperatures below 350° C., as shown by a solid line. When the ratio of NO:NO2 in NOx is 1:1, the NOx purifying facility can purify NOx most quickly. Further, it has been known that a high NOx purifying efficiency can be maintained at the catalyst temperature of approximately 200° C., as shown by a double-dashed line.
Generally speaking, a ratio of NO2:NOx in exhaust gases is 0:1, which is relatively small. Therefore, the NOx purifying efficiency is reduced as shown by the solid line in FIG. 5.
In order to overcome the foregoing problems, an oxidation catalyst supply may be added upstream of the NOx catalyst in the exhaust system in order to oxidize NO in exhaust gases to NO2, raise an NO2/NOx ratio, and supply NO2 to the NOx catalyst. This seems effective in keeping a NOx purifying efficiency at a high level.
However, in the foregoing case, the oxidation catalyst supply has to be positioned upstream of a reducing agent supply in the exhaust system. This inevitably enlarges a purifying facility. Therefore, it is difficult to secure a space for the oxidation catalyst supply. Further, in the case of a diesel engine, NO2 producing performance of the oxidation catalyst tends to be adversely affected by sulfur in exhaust gases resulting from sulfur components in fuel. Still further, purifying performance and durability of the catalyst may be affected by sulfur.
Therefore, the present invention aims at providing an engine control system which can increase an NO2 ratio in NOx and purify NOx efficiently even if the NOx purifying efficiency is low.